The present invention relates to a dynamic tracking reproduction apparatus, and more specifically to a reproduction apparatus suitable for a video tape recorder for recording video signals in helical scanning fashion, a DAT (digital audio tape recorder), etc.
In a recording and reproduction apparatus of helical scanning type such that a recording tape is wound around the circumference of a rotating head to increase the relative speed between the tape and the head, the recording tracks on the tape are not necessarily formed in a straight line but in a S-shaped curve in general. The cause of this S-shaped curve is that the working precision of the cylinder is not sufficient or the tension of the tape around the cylinder is not uniform, and their results in deterioration of reproduced pictures. To overcome these problems recently in the field of video tape recorders, in particular, there has been developed a reproduction apparatus in which a dynamic tracking mechanism for monitoring the positional relationship between the magnetic heads and the recording tracks to correct the offset between both during reproducing operation is incorporated for providing a higher reproduction quality.
In the above-mentioned video tape recorder, a plurality of reproducing heads are arranged; these reproducing heads are supported by an actuator in general composed of piezoelectric elements; and the reproducing heads are driven in the width direction of the video tracks of the magnetic tape by the actuator. Then, in a digital recording system for example, signals read from the reproducing heads are finally converted into binary signals of "1" and "0" as reproduced digital signals. Furthermore, DTF (dynamic track following) error signals indicative of offset rates of the heads from the video tracks, as pilot signals for later processing, are generated by a DTF error detector on the basis of the signals read from the heads. The generated DTF signals are given to the actuator driver to drive the piezoelectric elements in a direction such that the tracing offset can be eliminated. As described above, the head positions relative to the video tracks can be corrected to retain an optimum tracing operation.
In the prior art apparatus, however, there exists a problem in that jitter is easily generated. This is because the heads trace the video tracks in accordance with the DTF signals accompanied with a residual error determined by a servo system. The reason why the jitter is easily generated will be explained in more detail with reference to the attached drawings.
When two heads are used, the two heads trace two adjacent different tracks, individually. In FIG. 4, for instance, when the head la traces the track t2, the head 1b traces the tracks t1 and t3. Further, in general, an azimuth angle is previously determined between the two heads t1 and t2 in order to prevent cross talk between the adjacent tracks.
Therefore, as shown in FIG. 5, if the head la is located at the center of the track t2, the signal S matches the gap g. However, if the track is offset in the width direction of the track t2 as shown by a dashed line in FIG. 5, a time difference .DELTA..tau. is generated. The relationship between the positional offset and the time difference .DELTA..tau. is proportional as shown in FIG. 6. Accordingly, when the heads 1a and 1b trace the tracks along a zigzag line due to the servo control operation, as shown in FIG. 7, the reproduced signals are phase modulated on the basis of the frequency components of the zigzag motion, thus resulting in jitter. The above-mentioned jitter deteriorates the picture quality in the case of the analog recording video tape recorder, and the data error rate in the case of the digital recording video tape recorder.
Further, the above-mentioned residual error determined by the servo system is prominent in particular when a wobbling technique is adopted. In this wobbling technique, the heads are oscillated forcibly at a predetermined period to generate the pilot signals of large amplitude for improvement of error detection precision. Although being effective in the normal conditions, this wobbling technique emphasizes the above-mentioned jitter, because the magnetic heads are further oscillated forcibly along a zigzag line and therefore the residual error increases.
In summary, in the prior art dynamic tracking reproduction apparatus, there exists a problem in that the time axis of the reproducing signals fluctuate due to the zigzag motion of the heads on the tracks, thus it being impossible to generate reliable reproducing signals.